Explosive composition and process for making same



United ate This invention relates to a high explosive composition inpellet form having high thermal stability and high compressive strength.

In perforating oil wells at elevated temperatures, charges of pressedcyclonite (RDX) supported in metal or plastic containers have been used.The pressed cyclonite, besides requiring a supporting container becauseof lack of compressive strength, also lacks the requisite thermalstability in wells where the temperature exceeds 350 F. A need existsfor a high explosive pellet which has high degree of cohesion andcompressive strength, i.e., needs no metal or plastic supportingcontainer, and is stable at temperatures in the vicinity of 500 F.

In accordance with this invention, high explosive pellets having acompressive strength of at least 10,000 p.s.i. and stable at atemperature of 500 F. are provided which contain from 80 to 95 parts byweight of tetranitrodihenzo-1,3a,4,6a-tetraazapentalenc and from 20 to 5parts by weight of a binder which comprises an epoxy resin having morethan one epoxy group per molecule and a curing agent therefor which maybe pyromcllitic dianhydride (PMDA) or certain PMDA-polyhydric alcoholadduct compounds, the curing agent being present in an amount sufiicientto provide from 0.5 to 1.0 anhydride group for each epoxy group of theepoxy resin.

The tetranitrodibenzo-1,3a,4,6a-tetraazapentalene explosive used in myinvention and a suitable process for its preparation are described indetail in copending application Serial No. 116,922, filed June 8, 1961,in the name of R. A. Carboni and assigned to the present assignee. Thiscompound may be represented structurally as follows:

Tetranitrodibenzo-1,3a,4,6a-tetraazapentalene has two nitro groupsubstituents in any position on each benzene ring. There will obviouslybe numerous isomers of tetr-anitrodibenzo-l,3a,4,6a-tetraazapentalenebased upon the position of the nitro groups alone, which may be inortho, meta, or para relationship on each individual ring. The existenceof several of these isomers has been confirmed by infrared analysis, andall of the possible isomers in this regard are believed to be present toa greater or lesser extent in a synthesis product. For purposes of thepresent invention, there is no material distinction between any suchisomers, i.e., the explosive and thermal properties of these isomersappear to be quite similar and the explosive pellet of the presentinvention may include any of these isomers individually or may be acombination of any such isomers in any proportion. Actually, separationof the isomers is very diflicult because of the similarity in theirproperties, and synthesis of the composition would normally result in amixture of isomers, as noted above. This is perfectly satisfactoryinsofar as the explosive pellets of the present invention are concernedand the product may be used directly in this form.

The PMDA-polyhydric alcohol adduct compounds, which may be used inaccordance with the invention as the curing agent for the epoxy resin,are described as a class in detail in copending application Serial No.840,- 251, filed September 16, 1959, in the name of T. I. Hyde alongwith a suitable method of preparation. These compounds may berepresented structurally as follows:

o o11 HO-G 0 l 0:0

wherein ORO represents the nucleus of the polyhydric alcohol, i.e., thealcohol minus two of its hydroxyl hydrogen atoms. These materials areprepared by condensing at least one and one-half moles of PMDA with onemole of any of a wide variety of polyhydric alcohols, including suchalcohols as aliphatic glycols, such as 1,4-butanediol, ethylene glycol,ncopentyl glycol, and the like; ether glycols such as die thylene etherglycol, polyoxyethylene glycol, polyoxypropylene glycol, and the like;block-copolyrner glycols formed by the successive reaction of thecorresponding glycols with ethylene oxide and propylene oxide, or thelike; cyclic glycols; polyols, such as glycerol, 1,2,6-hexanetriol andthe like; diesters having terminal hydroxyl groups, such asbis(,8-hydroxyethyl) isophthalate and terephthalate; polyester materialshaving terminal hydroxyl groups, such as polyethylene terephthalate, andnatural products such as soybean oil and sugar, etc., containing two ormore hydroxyl functions per molecule. Any and all such condensationproducts are suitable in the invention as the curing agent for the epoxyresin as is PMDA itself.

Epoxy resins suitable in the invention are those having more than oneepoxy group per molecule, i.e., an epoxy equivalency greater than one.For example, the epoxy resins may be a g lycidyl polyether of apolyhydric phenol or alcohol, an alicyclic diepoxide resin, an aliphaticpolyepoxide resin, etc.

Although the amount of the epoxy resin-curing agent mixture combinedwith the tetranitrodibenzo-1,3a,4,6atetraazapentalene may range between20 and 5 parts by weight, for to 95 parts by weight oftetranitrodibenzo-1,3a,4,6a-tetraazapentalene, the preferred amount ofepoxy resin-curing agent mixture is 10 parts by weight for parts oftetran-itrodibenzo-1,3a,4,6a-tetraazapen talene.

The preferred amount of dianhydride compound is that which provides 0.8anhydride groups for each epoxide group, although as stated previously,a range of 0.5 to 1.0 anhydride group per epoxide group may be used.

The manner of mixing the tetranitrodibenzo-1,3a,4,6atetraazapentalenewith the epoxy resin-curing agent mix-j ture is not critical. For easein handling, the epoxy resin, which may be a viscous liquid or alow-melting solid, may be dissolved in or diluted with a convenientsolvent, e g., acetone, and then combined with the curing agent whichalso preferably is in solution form. When a PMDA-poly hydric alcoholadduct is used as the curing agent, the condensation of the PMDA and thealcohol may conveniently be carriedout in the presence of a reactionmedium solvent, such as acetone, methyl ethyl ketone, methyl isobutylketone, tertiarybutyl alcohol, and the like. This resulting solution ofthe condensation product, i.e., the adduct, may be mixed directly withthe epoxy resin or a solution of the epoxy resin in a solvent, such asacetone. Mixtures of solvents may also be used, e.g., acetone and methylethyl ketone. The solvent solution, of epoxy resin and the curing agentmay then be mixed with the solidtetranitrodibenzo-l,3a,4,6a-tetraazapentalene. The amount of solventused is not critical. However, the use of large amounts of solventaffords no economic advantage because of the need to remove the solventto form a free-flowing powder. The solvent may be removed from theexplosive-epoxy resin-curing agent mixture by conventional means, e.g.,vacuum drying, providing the temperature is maintained below thetemperature at which significant curing takes place (generally belowabout 50 F).

The powder mixture resulting from the removal of the solvent may becompressed into pellets immediately or, if desired, may be stored at lowtemperature, e.g., F. for extended periods to inhibit the curingreaction until the time desired. Pellets from both the powder mixturecompressed immediately after mixing and that compressed after storage at0 F. exhibited approximately the same compressive strength upon beingcured under identical conditions. Thus, in certain cases, it may beadvantageous to prepare the powder mixture beforehand and store themixture until the time it is desirable to form pellets.

In order to provide the requisite density and compressive strength, theamount of compressive pressure required to form the pellets will dependupon the amount of binder used. When the weight percent of binder usedis low, e.g., approximately a compressive pressure of approximately50,000 psi. is generally used to obtain a pellet, which when cured, willhave compressive strength of at least about 10,000 psi. When the weightpercent of binder used is high, e.g., 15%, a compressive pressure as lowas 15,000 psi. is suitable to obtain a pellet, which when cured, willhave a compressive strength of at least 10,000 p.s.i. Of course, highercompressive pressures, if desired, may be applied without anydetrimental effect; however, no economic advantage entails therefrom.Generally, when a composition containing the preferred amount of binder,by weight, is to be pelleted, the compressive pressure applied will beat least 35,000 p.s.i.

The temperature at which the pellets are cured may range from roomtemperature to about 400 F. Of course, curing at room temperaturerequires a period of several days to effect substantial crosslinking;whereas, at elevated temperatures, e.g., 400 F., substantialcrosslinking occurs in several hours. The optimum curing schedule(time-temperature relationship) has been found to be about 10 hours at400 F.

In the following examples which illustrate the invention, a Dillon ModelL Universal Testing Machine with a compression cage, 010,000-pounddynamometer, and powered with an air motor was used to compress thepellets. The compressive strength was determined according to ASTM TestMethod D595-54, as described in ASTM Standards, 1953 edition, Part 9,pages 244-251.

In all of the ensuing examples, the epoxy resin employed was a liquidepoxy resin which had an approximate molecular weight of 390, an epoxidevalue of 0.52 per 100 grams, and a viscosity, at 25 C., of about 100 toabout 160 poises. This resin is a glycidyl polyeth of a i dihydricphenol formed by the condensation of epichlorohydrin and4,4"isopropylidene diphenol, and is available commercially as Epon" 828,manufactured by Shell Chemical Company.

The PMDA-polyhydric alcohol adduct used as a curing agent for the epoxyresin in the examples, unless otherwise indicated, was prepared byrefluxing, under a blanket of dry nitrogen for a few hours, a mixture of30 parts of PMDA, 25 parts of 2,2-bis[4--(Z-hydroxyethoxy)phenyl]propane (which is available commercially as Dow Resin X-2635manufactured by the Dow hemical Company), and 45 parts of methyl ethylketone as a solvent. The molar ratio of PMDA/ alcohol was 2/ 1. Theadduct obtained may be represented as follows:

The parts in the examples are parts by weight.

i (EH3 Example 1 A free-flowing powder was prepared by mixing 36 partsof particulate tetranitrodi'benzo-1,3a,4,6a-tetraazapentalene having anaverage particle size of about microns with a premixed solution of 2.2parts of the epoxy resin dissolved in 27 parts of acetone and 3.3 partsof the PMDA-polyhydric alcohol adduct solution (containing 1.8 parts ofsolids). This combination provided 0.8 anhydride group for each epoxygroup. The mixture was vacuum dried at a temperature below 50 C. toremove all traces of acetone and kctonc.

Two and one-half-part portions of the thus prepared powder wereimmediately compressed without application of heat in a cylindrical moldapproximately /2 inch in diameter and /2 inch in length under 40,000p.s.i. pressure for a 30-second dwell time. The pellets were curedat 400F. for a period of 18 hours. The cured pellets had a density ofapproximately 1.63 g./ec. and representative pellets exhibitedcompressive strengths of approximately 13,000 psi.

The detonation velocity of the pellet was 7,200 m./sec. When smallquantities of the uncured and cured explosive composition were subjectedto the standard impact sensitivity test, the explosive drop tests were,respectively, 51 and 55 inches. In the impact sensitivity tests, aS-cilogram Weight is dropped upon the explosive from various heights.Drop test results are reported in inches which correspond to thedistance which the Weight must fall to cllcct initiation. Thoseexplosives initiated only when the weight falls considerable distances,e.g., 40 to 56 inches, are of low impact sensitiveness.

When a portion of the molding powder was allowed to remain standing for1 day before he ig compressed into a pellet and cured according to theprocedure described above, the pellets had a density of 1.61 g./cc. anda compressive strength of 11,500 p.s.i.

When a small portion of the molding powder was allowed to remainstanding 6 days before being compressed and cured according to theprocedure described above, the pellets had a density of 1.61 g./cc. anda compressive strength of 10,500 p.s.i.

Example 2 500-n1illilitcr flask, was vacuum dried. A portion of thepowder nnxture was refrigerated at 0 F. overnight and then compressedaccording to the procedure described in Example 1. Pellets cured at 400F. for a period of 4 hours had a density of 1.62 g./cc. and acompressive strength of 10,500 p.s.i. Pellets cured at 400 F. for aperiod of 8 hours had a density of 1.62 g./ cc. and a compressivestrength of 11,500 p.s.i. Pellets cured at 400 F. for a period of 18hours had a compressive strength of 11,000 p.s.i. and lost only 0.4%weight.

A portion of the molding powder was stored at F. for a period of 13 daysand then compressed into pellets according to the procedure described inExample 1. Pellets cured at 400 F. for a period of 18 hours had acompressive strength of 11,000 p.s.i.

Example 3 The procedure of Example 2 was used for preparing afree-flowing powder except that 36 parts of particulatetetranitrodibenzo-l,3a,4,6a-tetraazapentalene, 2.8 parts of the epoxyresin, and 1.2 parts of PMDA were used. This combination provided 0.8anhydride group for each epoxide group. A portion of the mixture wasimmediately compressed as described in Example 1. Pellets cured at roomtemperature fora period of days had a density of 1.66 g./-cc. and acompressive strength of 12,000 p.s.i. When these pellets were curedfurther at 400 F. for a period of 18 hours, the density decreased to1.61 g./cc. and the compressive strength was only slightly lowered to11,500 p.s.i.

When the above cured pellets were heated in an oil bath at graduallyincreasing temperatures to determine the thermal stability, the pelletsremained stable, that is did not show evidence of internal heating untila temperature of 520 F. was attained and only fumed oil (did notdetonate) when the temperature reached 550 F.

Example 4 A free-flowing powder was prepared by mixing 127.5 parts ofparticulate tetranitrodibenzo-l,3a,4,6a-tctraazapentalene having anaverage particle size of about 100 microns with a premixed solution of15.6 parts ol the epoxy resin and 6.9 parts of PMDA in 7080 parts ofacetone. This combination provided 0.8 anhydride group for each epoxygroup. The acetone was evaporated by heating the stirred mixture on asteam bath until a friable solid resulted. The solids then were screenedthrough a silk cloth to eliminate any lumps and vacuum dried at 50 C.for 1 hour.

Two and one-half-part portions of the thus prepared free-flowing powderwere immediately compressed without application of heat in a cylindricalmold approximately /2 inch in diameter and /2 inch in length for :asecond dwell time. The characteristics of pellets compressed at variouspressures and cured at 400 F. for 16 hours are given in the followingtable.

Compres- Density of Compression Pres- Cured Pelsive sure (p.s.i.) lets(g./cc.) Strength (p.s.i.)

Pellets prepared and cured as above were unaffected by subjection to atemperature of 500 F. for a period of 18 hours.

Pellets prepared and cured as above were initiated by an electricblasting cap containing 3 grains of an igniter mix 25/ 25 parts byweight of smokeless powder/potassium perchlorate/and the lead salt ofdinitro-ocresol, respectively), 3 grains of lead azide as a primercharge, and 3 grains of pen-taerythritol tetranitrate as the base chargeor, alternatively, a thin-bottomed aluminum sheathed cap containing anigniter bead (75/20/5 parts by weight of the lead salt ofdinitro-o-cresol/ potassium perchlorate/selenium, respectively), 2grains of lead azide as a primer charge, and 2 grains of pentaerythritoltetranitrate as the base charge. The detonation velocity of the pelletwas 6,650 meters per second.

Example 5 A free-flowing powder was prepared as described in Example 4,except that 95 par-ts of tetranitrodibenzol,3a, 4,6a-tetraazapentalene,1.5 parts of PMDA, 3.5 parts of the epoxy resin, and 32 parts of acetonewere used. Pellets compressed at 50,000 p.s.i. and cured at 400 F. for16 hours had a compressive strength of 10,000 p.s.i. and a density of1.62 g./cc.

While the invention has been described and illustrated in the foregoingspecification, it will be readily apparent to those skilled in the artthat many variations may be made in the compositions and proceduresdisclosed without departing from the spirit or scope of the invention.It is intended, therefore, to be limited only by the following claims.

We claim:

1. A new explosive composition comprising a mass of hard, tough,compact, thermally-stable pellets containing from to parts by weight of:

and from 20 to 5 parts by weight of a mixture of an epoxy resin havingmore than one epoxide group per molecule and a dianhydride curing agentfor the epoxy resin selected from the group consisting of pyromelliticdianhydride and the condensation products of pyromelliti'c dianhydrideand polyhydric alcohols in a molar ratio of at least 1.5:1, the amountof dianhydride curing agent being sufiicient to provide from 0.5 to 1.0anhydrid-e group for each epoxy group of the epoxy resin, and curing theepoxy resin with the said curing agent.

3. A process as in claim 2 wherein the curing is effected by heating thepellets to a temperature within the range of about 60 to 400 F.

4. A process for preparing a novel explosive composition which comprisesmixing from 80 to 95 parts by weight of:

than one epoxide group per molecule and a dianhydride curing agent forthe epoxy resin selected from the group 7 consisting of pyromelliticdianhydride and the condensation products of pyromellitic dianhydrideand polyhydric alcohols in a molar ratio of at least 1.5:1, the amountof dianhydride curing agent being sufiicient to provide from 0.5 to 1.0anhydride group for each epoxy group of the epoxy resin, removing theorganic solvent at a temperature below about 50 F. to obtain afree-flowing powder,

5 the range of about 60 to 400 F.

No references cited.

1. A NEW EXPLOSIVE COMPOSITION COMPRISING A MASS OF HARD, TOUGH,COMPACT, THERMALLY-STABLE PELLETS CONTAINING FROM 80 TO 95 PARTS BYWEIGHT OF: